1. Field of the Invention
The present invention relates to termination circuits and methods therefor. More particularly, the present invention relates to termination circuits that provides fast and efficient clamping for signals transmitted via transmission lines in electronic systems.
2. Description of Relevant Art
In the design and implementation of electronic systems (such as digital computers, consumer/commercial electronic devices, or the like), particularly those employing integrated circuits, undesired transmission line effects are of a particular concern. As signals travel down transmission lines, e.g., traces on a printed circuit board, reflections may occur on the lines. The reflections are due to, for example, mismatched impedances between the driver circuit and the line, which may cause the signal to reflect back and forth, giving rise to ringing. These reflections and other undesired transmission lines effect are often exacerbated as the operating speed of the signal increases. If left uncorrected, the reflections may cause the signal's voltage to swing outside of the defined "0" or "1" voltage levels, thereby causing the receiving device to incorrectly interpret the signal received and generate erroneous results.
To address the impedance mismatch between the driver (or receiver) circuit and the transmission line, a variety of techniques has been tried in the prior art. FIG. 1A illustrates a resistor-terminated approach wherein a resistor 102 is inserted between the end of the transmission line and ground or alternately to the power supply. Resistor 102, whose value is chosen to provide impedance matching to the transmission line 106 so as to avoid reflections, thereby preventing the voltage on transmission line 106 to swing outside of the signal's defined operating ranges.
FIG. 1B illustrates an alternative series resistor-terminated approach wherein impedance matching resistor 152 is inserted between driver circuit 104 and transmission line 106. Impedance matching resistor 152 provides impedance matching to transmission line 106. This system relies on a half voltage signal propagating down the line, being double in magnitude and propagating back to the driving end, at which point it sees a properly terminated line and stabilizes.
While the resistor-terminated approach proves suitable for some systems, there are disadvantages. For example, the use of an impedance matching resistor attenuates the signal, which lowers noise immunity while dramatically increasing power dissipation. To illustrate, the presence of resistor 102 forms essentially a resistor divider circuit between the characteristic impedance of line 106 and resistor 102, thereby lowering the signal's voltage level at the receiving circuit.
Further, the presence of resistor 102 increases the power dissipation, which increases the load on the driver circuit and causes additional heat to be generated as well as more power from the system's supply. The presence of resistor 152 in FIG. 1B creates a situation in which the input to any other device which might be connected to the transmission line sits at half amplitude, an undesirable condition while the line is settling. Still further, it is often difficult to provide impedance matching for transmission lines whose characteristic impedance may change with the system's configuration. By way of example, transmission lines coupling with a computer's memory system may terminate at a plurality of memory slots. Depending on the amount of memory provisioned, some or all of these slots may be occupied. As can be appreciated from the foregoing, the characteristic impedance of these transmission lines may depend on the number of memory slots occupied. The variable impedance of these transmission lines makes it difficult to address impedance matching using resistors.
Diode-terminated matching circuits represent another approach to minimizing reflections on transmission lines. FIG. 2 illustrates a simplified diode-terminated matching circuit 200, comprising diodes 202 and 204. As shown in FIG. 2, diode 202 is coupled between common terminal 206 and one rail voltage (ground in this case). Diode 204 is coupled between common terminal 206 and the other rail voltage (V.sub.DD in this case). As the signal travels down line 106, reflections increase the voltage thereon, causing the diodes to turn on to clamp the voltage on line 106 at the prescribed clamping voltages. In the case of FIG. 2, the clamping voltages are V.sub.DD +V.sub.TD and ground -V.sub.TD wherein V.sub.TD represents the forward drop voltage of the diode. To ensure quick turn off of the diodes when the line's voltage is within the prescribed clamping voltages, Schottky diodes are typically employed.
In the diode-terminated approach, impedance matching is not critical. Accordingly, the diode-terminated approach is more suitable for transmission lines whose characteristic impedance may be variable. It is believed that such approaches have been in common use since the late 1960's. As the operating voltages of electronic devices decrease, however, diode-terminated clamping circuits prove inadequate. By way of example, it is contemplated that microprocessors or memory circuit employing 0.1 micron technology may operate with operating voltages as low as 1 volt. Since the forward drop voltage of a Schottky diode is typically around 0.6 V, the diode-terminated clamping circuit will not start to clamp until the voltage on the transmission line swings above 1.6 V (V.sub.DD +V.sub.TD) or below -0.6 V (-V.sub.TD). In other words, the voltage on the transmission line may vary by up to 60% before clamping begins. Such a wide disparity between the clamp voltage and the operating voltage makes this application of diodes ineffective.
Although manufacturers of electronic systems have long desired an easy-to-implement termination circuit design that can provide efficient clamping for modem high speed, low voltage signals, most of the attempts have been in the direction of improving the diode-terminated approach (e.g., by attempting to reduce the forward bias voltage of the diodes in the diode-terminated solution) or the resistor-terminated approach. This is because the task of addressing impedance mismatches at the board level is typically assigned to VLSI digital engineers, who have more familiarity with digital systems techniques than the complexities of analog line terminations. If analog engineers are assigned to the task, they typically have more familiarity with analog circuitry, e.g., diodes/resistors, than with VLSI design principles. By way of example, when the diode drops are too high, the engineers typically turn to tuning the termination system with resistors. In doing so, they increase power dissipation or in other ways impact system performance as indicated above.
With current fabrication technology, a reliable zero voltage forward bias diode has not been found. Accordingly, current diode-terminated designs continue to prove unsuitable for use with modern low voltage circuits. Moreover, even if such a diode could be obtained, the diode-terminated clamping circuit (as well as the resistor-terminated clamping circuit for that matter) cannot be easily integrated into modern CMOS (complementary metal oxide semiconductor) receiving or driving circuits, such as microprocessor or memory circuits. Typically, these approaches require that the termination circuit be implemented as a separate, stand-alone chip. For designs that are form-factor limited, e.g., small or portable electronic systems, this requirement is highly disadvantageous since it requires additional space on the circuit board and increases costs.
In view of the foregoing, there are desired improved termination circuits and methods therefor that advantageously provide fast and efficient clamping for signals transmitted via transmission lines in electronic systems, particularly signals having low operating voltage ranges.